U.S. patent number 5,004,501 [Application Number 07/352,569] was granted by the patent office on 1991-04-02 for two phase cement mixture, particularly suitable for othopaedics.
This patent grant is currently assigned to Tecres Spa. Invention is credited to Basilio M. De Bastiani, Giovanni Faccioli, Bruno Magnan, Renzo Soffiatti.
United States Patent |
5,004,501 |
Faccioli , et al. |
April 2, 1991 |
Two phase cement mixture, particularly suitable for othopaedics
Abstract
The invention relates to a two phase cement mixture which is
particularly suitable for orthopaedic use, in which the solid phase
comprises a polymer, polymetyl methacrylate, and a catalyst,
benzoyl peroxide, while the liquid phase comprises a monomer,
monomethyl methacrylate, an accelerator, N-N-dimethyl-p-toluidine
and hydroquinone. The said polymetyl methacrylate is in powder form
with particles of a spherical shape which are of a suitable
particle size. Fluoride in form of salt can be added to the said
solid phase, realizing fluoride ions, making them available to the
part of the bone with which the said mixture is in contact when is
used to attach a prothesis to a bone.
Inventors: |
Faccioli; Giovanni (Monzambano,
IT), De Bastiani; Basilio M. (Montegrotto,
IT), Magnan; Bruno (Verona, IT), Soffiatti;
Renzo (Nogara, IT) |
Assignee: |
Tecres Spa (Bussolengo,
IT)
|
Family
ID: |
11326322 |
Appl.
No.: |
07/352,569 |
Filed: |
May 16, 1989 |
Foreign Application Priority Data
|
|
|
|
|
Jun 1, 1988 [IT] |
|
|
84950 A/88 |
|
Current U.S.
Class: |
106/35; 523/117;
524/415; 524/436; 524/522; 524/523; 523/116; 524/401; 524/428;
524/438 |
Current CPC
Class: |
A61L
24/0015 (20130101); A61L 24/06 (20130101); A61L
24/06 (20130101); C08L 33/12 (20130101); A61L
2430/02 (20130101); A61L 2300/106 (20130101); A61L
2300/622 (20130101) |
Current International
Class: |
A61L
24/00 (20060101); A61L 24/06 (20060101); C09K
003/00 (); A61F 005/04 (); A61F 002/00 (); C08K
003/00 () |
Field of
Search: |
;128/92YQ
;524/401,415,428,436,438,522,523 ;523/116,117 ;106/35 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Morgan; Kriellion
Attorney, Agent or Firm: Ladas & Parry
Claims
We claim:
1. A two phase cement mixture which is particularly suitable for
orthopaedic uses, comprising a solid powder phase and a liquid
phase, wherein the solid phase comprises a polymer, polymethyl
methacrylate (--(C.sub.5 H.sub.8 O.sub.2).sub.n --) and a catalyst,
benzoyl peroxide (C.sub.14 H.sub.10 O.sub.4), while the liquid
phase comprises a monomer, monomethyl methacrylate C.sub.5 H.sub.8
O.sub.2, an accelerator, N-N-dimethyl-p-toluidine C.sub.9 H.sub.13
N and a stabilizer, hydroquinone, characterized in that:
(a) the polymer consists of particles of spherical shape only;
(b) the spherical particles are present in proportions with
diameters ranging up to 87 .mu.m; and
(c) particles having diameter up to 0.90 .mu.m comprise between
0.6% and 2.0% of the polymer.
2. A two phase cement mixture as claimed in claim 1, in which the
amount of liquid phase required to react with a standard 40 g dose
of solid phase is 14 ml, and in which the polymer, having particles
of a spherical shape only, consists of:
spheres of diameter up to 0.90 .mu.m, in a percentage lying between
0.60 and 2.00%,
spheres of diameter 0.91 .mu.m to 3.70 .mu.m, in a percentage lying
between 0.80 and 2.00%,
spheres of diameter 3.71 .mu.m to 10.50 .mu.m, in a percentage
lying between 3.00 and 5.00%,
spheres of diameter 10.51 to 25.00 .mu.m, in a percentage lying
between 15.00 and 19.00%,
spheres of diameter 25.01 .mu.m to 51.00 .mu.m, in a percentage
lying between 45.00 and 55.00%,
spheres of diameter 51.01 .mu.m to 87.00 .mu.m, in a percentage
lying between 22.00 and 28.00%, and
the total percentage polymer in the powder equal or smaller in
diameter than 87.00 .mu.m being equal to 100%.
3. A two phase cement mixture as claimed in claim 1 in which an
amount of floride between 3.0 and 9.0% in the form of a fluoride
salt which is capable of releasing fluoride ions, F.sup.-
gradually, making them available to bone tissue is added to the
said mixture.
4. A two phase cement mixture as claimed in claim 3, in which at
least one of the following salts are added to the solid phase in
the proportions specified: sodium fluoride (NaF), ammonium fluoride
(NH.sub.4 F), sodium monofluoride phosphate (Na.sub.2 PO.sub.3 F),
sodium silicofluoride (Na.sub.2 SiF.sub.6), tin fluoride
(SnF.sub.2), potassium fluoride (KF), magnesium fluoride
(MgF.sub.2), lithium fluoride (LiF), zinc fluoride (ZnF.sub.2),
potassium hexafluorophosphate (KPF.sub.6), ammonium
hexafluorophosphate (NH.sub.4 PF.sub.6), sodium hexafluorosilicate
(Na.sub.2 SiF.sub.6).
5. A two phase cement mixture as claimed in claim 2, in which an
amount of fluoride between 3.0 and 9.0% in the form of a fluoride
salt which is capable of releasing fluoride ions F.sup.- gradually,
making them available to the bone tissue, is added to the said
mixture.
Description
The invention relates to a two phase cement mixture which is
particularly suitable for orthopaedic uses, having a solid phase
comprising predominantly polymers and a liquid phase comprising
predominantly monomer. The said phases are then joined together at
the time of use to form a resin of plastic consistency which
hardens in the course of time.
The said mixture, commonly known as bone cement, is known for use
in orthopaedic surgery to provide a firm attachment for prostheses
of various types to a variety of points on the uman skeleton. The
term "cement" may incorrectly suggest an adhesive. In fact its
function is instead to fill the spaces existing between the
prosthesis, which is generally of metal, and the cavity in the bone
prepared for its implantation.
This filling effect, associated with minimum physical expansion of
the resin during polymerisation, provides a mechanical anchorage
and a perfect fit between the implant and the bone. The best known
use of this bone cement, to which we will refer without thereby
restricting the scope of the invention, is that associated with the
application of hip prostheses.
This surgical technique will now be described in principle in order
that the invention may be better understood.
Once the need to replace the head of the femur with a prosthesis
has been diagnosed, access is gained to the head by surgery and it
is exposed so that it can be resected. The bone cavity is then
bored out so that the cavity fits the shape of the prosthesis.
The cement is then prepared by combining the liquid phase with the
solid phase and mixing the two until a plastic past is obtained.
The cement so obtained is placed in the bone cavity and while it is
still plastic the prosthesis is embedded in it and positioned
accurately. There then is a wait of ten to fifteen minutes for the
cement to harden and the femur is then repositioned with the new
head in the correct position.
A similar procedure is used to position a cup prosthesis attached
to the joint component of the pelvis. The cup made surgically is
then closed completing the operation.
Because the orthopaedic cement is placed in direct contact with
bone tissue, the chemical composition of the latter will now be
described.
Bone tissue has two components: an inorganic component, also known
as the mineral component, which forms the rigid framework of the
tissue, and an organic component, also known as the biological
component, which represents the "living" part of the structure.
The mineral component consists of calcium hydroxyapatite which
precipitates in the tissue in the form of crystals, followed by a
biochemical reaction which takes place in the organic matrix of the
tissue under particular environmental conditions (pH,
concentration, etc.) and in the presence of enzymes.
The organic component of the structure can be regarded as a
connective tissue, that is a set of active cells specialised to a
greater or lesser extent immersed in a matrix produced by the cells
themselves. It is in this matrix, which is produced by the
osteoblasts, i.e. the cells which specialise in the formation of
the bone tissue, that the mineral crystals which give rise to the
hydroxyapatite precipitate.
Wthen it is mature the bone tissue constructed in this way is
organised into sheets which can form bony trabeculae or more
compact bone tissue, also known as cortical tissue.
The cells contained within the metabolically stationary bony tissue
are the osteocytes, while the cells responsable for the destruction
and reabsorption of the tissue are the osteoclasts.
Both the osteoblasts and the osteoclasts are metabolically active
cells and are subject to many controls of both a physiological and
an artificially induced nature, the latter being of the chemical,
biological or physical type transmitted to the above-mentioned
cells by chemical substances such as hormones or drugs, or by
physical stresses of the either a mechanical or electrical or
electromagnetic type. It has been found clinically following
orthopaedic implant prostheses cemented with acrylic resins that
the use of known bone cements has the following disadvantages.
In a certain number of cases detachment or aseptic mobilisation of
the implant occurs after a variable period of time.
This phenomenon is the most important complication of this surgical
technique and is undoubtedly the factor which determines the result
of the entire operation.
This detachment takes place at the bone-cement interface and take
form of localised reabsorption of the bone tissue around the
implant, with the replacement of this tissue by a reactive fibrous
tissue, which may even be some thickness, which gives rise to
mobilisation of the implant.
World literature attributes a primary role in the mechanism giving
rise to detachment to the high temperature reached by the paste
when it is hardenig as a result of the exothermic reactions
produced by polymerisation. The temperature reached by the paste
during the polymerisation varies, in clinical use, from 70.degree.
to 90.degree. depending on the cement used, as described by B.
Mjoberg, A. Rydholm et al. in the paper "low versus high viscosity
bone cement" published in Acta Ortop Scand. 58, 106-108 in
1987.
The paste at high temperature in contact with the internal bone
surface of the cavity produces scalding of the bone tissue which in
turn gives rise to the formation of a necrotic-fibrous membrane
consisting of dead cells which completely surrounds the cement mass
introduced into the bone.
This membrane increases continually with the passage of time.
Following repeated stressing of the prosthesis caused by the load
transferred to it, this membrane is compressed and flattened thus
giving rise to play between the cement-prosthesis implant and the
bone. This play allows the cemented prosthesis an increasing amount
of the movement which initiates and amplifies wear of the material
until the reconstructed joint fails.
In such cases cardio-respiratory depression due to the excessive
amount of the liquid monomer coming into contact with the bone
tissue occurs immeediatly after the cement is introduced into the
bone cavity.
This depression makes it necessary to administer suitable drugs to
the patient while still undergoing surgery in order to avoid
possible cardio-respiratory collapse. This effect can be reduced
somewthat by reducing the amount of the liquid monomer which is
needed to form the correct paste.
The use of fluoride salts in osteoporotic syndromes, that is
pathological rarefaction of the bone structure is based on
observations made by Dr. Rotholm on workers occupationally exposed
to the inhalatio or ingestion of large amounts of fluorine
compounds.
The mechanisms of the action of the fluorides on bone tissue can be
controlled and reproduced, as shown in work by various authors in
the review "Fluoride in medicine", et T. L. Vischer, of 1970.
The action of fluoride is explained, by a double mechanism, one of
a biochemical nature and the other biological.
In the biochemical mechanism there is incorporation of the fluoride
ion into the mineral structure of the bone, with a consequent
increase in the dimensions of the hydroxyapatite cristal. This
causes the hydroxyapatite to become less water-soluble and
increases the binding force between the organic matrix and these
cristals with a consequent improvement in the more mechanical
properties of the bone structure. An increase in the crystallinity
index has been determined experimentally by measurements using an
infrared spectrometer.
In the biological mechanism on the other hand there is direct
stimulation of the osteoblasts, which can be detected as an
increase in their number and activity, and by transitory
morphological changes in them, and therefore with the consequent
new production of uncalcified bone matrix. The histomorphometric
consequence of this succession in an increase in the volume of the
trabeculae which can reach 20% in the first year of the
treatment.
With reference to the biochemical mechanism it should be noted that
the fluoride ion is rapidly captured by the bone tissue and
incorpored into the mineral structure of the hydroxyapatite where
it displaces the hydroxil group (--OH) forming fluorohydroxyapatite
(FAP). Fluoride ions can displaces up to 25% of the hydroxyl
radicals in the hydroxyapatite with a maximum saturation
concentration in bone of 20,000 to 35,000 parts per million,
equivalent to 40-70 mg of sodium fluoride (NaF) per gram of bone
tissue.
This value however represents the theoretical maximum corresponding
to chemical saturation of the bone.
The actual values which can be measured in the course of oral
treatment or in the case of occupational fluorosis are obviously
very much lower because of the state of equilibrium which is set up
between the amount taken up, the amount eliminated by excretion
from the kidneys, the amount captured by the bone and the amount
released through the effect of the half-life of the fluorine in the
bone, which is about two years.
This systemic or oral administration of the fluoride has the
following disadvantages.
When the drugs is taken in high doses it can cause excessive
accumulation throughout the skeleton, with consequent pathological
fluorosis of the bone and toxic effects in some of the patient's
organs which can make it necessary to reduce the dosage of the
drug, and may also produce an unacceptable level of local
accumulation at the site of the implant.
The object of the invention is at least to minimise the
abovementioned disadvantages.
According to the present invention there is provided a two phase
cement mixture which is particularly suitable for orthopaedic uses,
characterised in that the solid phase comprises a polymer,
polymetyl methacrylate (--(C.sub.5 H.sub.8 O.sub.2).sub.n --) 97%
and a catalyst, benzoyl peroxide (C.sub.14 H.sub.10 O.sub.4) 3%,
while the liquid phase comprises a monomer, monomethyl methacrylate
(C.sub.5 H.sub.8 O.sub.2) 99.10%, an accelerator,
N-N-dimethyl-p-toluidine (C.sub.9 H.sub.13 N ) 0.89% and a
stabiliser, hydroquinone approximately 20 parts per million, the
amonth of liquid phase required to react with a standard 40 g dose
of solid phase being 14 ml, and in which the said powder polymer,
having particles of a spherical shape only, consists of:
spheres of diameter up to 0.90 .mu.m, in a percentage lying between
0.60 and 2.00%,
spheres of diameter 0.91 .mu.m to 3.70 .mu.m, in a percentage lying
between 0.80 and 2.00%,
spheres of diameter 3.71 .mu.m to 10.50 .mu.m, in a percentage
lying between 3.00 and 5.00%,
spheres of diameter 10.51 .mu.m to 25.00 .mu.m, in a percentage
lying between 15.00 and 19.00%,
spheres of diameter 25.01 .mu.m to 51.00 .mu.m, in a percentage
lying between 45.00 and 55.00%,
spheres of diameter 51.01 .mu.m to 87.00 .mu.m, in a percentage
lying between 22.00 and 28.00%, the total percentage polymer in the
powder passing the 87.00 .mu.m sieve being equal to 100%.
Preferably the powdered polymethyl methacrylate is in particles of
sferical shape only consists of:
spheres of diameter up to 0.90 .mu.m, in a percentage lying between
0.60 and 2.00%,
spheres of diameter 0.91 .mu.m to 3.70 .mu.m, in a percentage lying
between 0.80 and 2.00%, in which the spheres having a diameter
which passes the 1.10 .mu.m sieve represent at least 30% of the
totality of the said spheres and the spheres having a diameter
passing through the 2.20 .mu.m sieve represent at least 97% of all
the said spheres,
spheres of diameter 3.71 .mu.m to 10.50 .mu.m, in a percentage
lying between 3.00 and 5.00%, in which the spheres having a
diameter passing through the 9.00 and 10.50 .mu.m sieves represent
at least 25 and 27% of all the said spheres,
spheres of diameter 10.51 .mu.m to 25.00 .mu.m, in a percentage
lying between 15.00 and 19.00%, in which the spheres having a
diameter passing through the 21.00 and 25.00 .mu.m sieves represent
at least 21 and 29% of all the said spheres,
spheres of diameter 25.01 .mu.m to 51.00 .mu.m, in a percentage
lying between 45.00 and 55.00%, in which the spheres having a
diameter passing through the 51.00 and 43.00 .mu.m sieves represent
at least 28% of all the said spheres,
spheres of diameter 51.01 .mu.m to 87.00 .mu.m, in a percentage
lying between 22.00 and 28.00%, in which the spheres having a
diameter passing through the 61.00 and 73.00 .mu.m sieves represent
at least 50 and 33% of all the said spheres,
the total percentage polymer in the powder passing the 87.00 .mu.m
sieve being equal to 100%.
An amount of floride between 3.0 and 9.0% in the form of a fluoride
salt which is capable of releasing fluoride ions F.sup.- gradually,
making them available to the bone, is added to the said
mixture.
Preferred fluoride salts are: sodium fluoride (NaF), ammonium
fluoride (NH.sub.4 F), sodium monofluoride phosphate (Na.sub.2
PO.sub.3 F), sodium silicofluoride (Na.sub.2 SiF.sub.6), tin
fluoride (SnF.sub.2), potassium fluoride (KF), magnesium fluoride
(MgF.sub.2), lithium fluoride (LiF), zinc fluoride (ZnF.sub.2),
potassium hexafluorophosphate (KPF.sub.6), ammonium
hexafluorophosphate (NH.sub.4 PF.sub.6), sodium hexafluorosilicate
(Na.sub.2 SiF.sub.6).
The solid phase and fluoride salt may be marketed in separate packs
or jointly in the same pack.
Research into the phenomenon of detachment has resulted in
identification of the following factors which give rise to
detachment:
the chronic inflammatory reaction set up by the remains of the
materials used for the prosthesis,
mechanical yieding of the cement and other materials used due to
the considerable and cyclically variable loads to which the
materials are subjected during daily use of the prosthesis,
lesion of the bone tissue caused by direct contact with the acrylic
resin during polymerization, due to the large amount of the heat
released by the resin following an exothermic polymerisation
reaction; as described in the literature the threshold of heat
damage for biological structure is around 70.degree. C., above this
threshold structure are irreversibly denatured,
biological reaction of the bone tissue of a self-destructive or
catabolic nature caused by abnormal biomechanical stimuli due to
load on the implanted prosthesis acting at the cement-bone
interface.
When the abovementioned causes of detachment had been identified
efforts were made to prevent or at least limit these phenomena
through the development of a bone cement having better mechanical
strength, a low heat of polymerisation, below the threshold for
heat demage to biological structures, and associated with fluoride
salts which can release fluoride ions locally in a sufficient and
harmless concentration in a gradual way over a long period of the
time.
The main advantage offered by the invention consists of the fact
that as a result of precise selection of the particle size and
shape of the particles forming the solid phase of the bone cement
the amount of liquid monomer required to cause a complete dose of
cement powder to react completely and thus to obtain accurate and
homogeneous mixing is reduced dramatically in comparison with the
amounts usually used.
Bearing in mind that the amount of heat produced in the
polymerisation reaction is proportional to the amount of liquid,
this reduction in the amount of the liquid produces a proportional
decrease in the amount of heat released by the polymerisation
reaction, which for a given mass of cement is reflected in a fall
in the absolute polymerisation temperature.
This temperature in thus kept below 55.degree. C. in comparison
with the 70.degree./90.degree. C. reached in the clinical use of
known cements, without this having any adverse effect on the
mechanical strength characteristics of the product.
A further advantage, again due to the reduction in the amount of
liquid phase used to obtain the cement paste, arises from the fact
that the probability of the risk of cardio-respiratory collapse in
the patient following administration of the liquid monomer is
reduced.
Another advantage, confirmed by experiments performed on samples of
cement according to the invention in a laboratory working to
British Standard ISO/DP 5833/1, is due to the improvement in the
mechanical properties of the cement itself in comparison with the
corresponding properties of known cements obtained in the same
tests and shown in Table II.
Yet another advantage is brought about by the addition of fluorine,
in the form of salt, directly into the bone cement. The
administration of fluorine in situ, in contact with the bone which
is to receive it, eliminating the disadvantages of systemic
administration, in fact appreciably improves the availability of
the fluorine to the bone, making it available for a longer period
of time. It is in fact known that the amount of release is
associated with various factors such as: the size of the molecule
of the additive, the temperature and hydration of the environment
and the extent of the area of contact between the cement and the
environment.
It has also been found that the amount of release is greater in the
presence of:
little or no chemical bonding between the additive and the polymer
forming the cement,
small size of the additive molecule,
high temperature,
large area of contact between the polymer and the bone tissue,
biological liquids in contact with the polymer.
In the light of these studies, and beyond the restricted field of
application of antibiotics, it has been concluded that sodium
fluoride is a particularly suitable substance for local release in
an slow and controlled manner.
Sodium fluoride in fact has following properties:
it contains the greatest amount of fluoride per unit weight,
the molecule is simple and of fairly small size,
it is not possible for chemical bond to form between carbon atoms
and fluoride ions, and therefore between the polymer and the added
fluoride,
the diffusion of the fluorine in ionic form from the cement to the
external environment is due to contact erosion of the water surface
in the environment which dissolve the sodium fluoride, extracting
Na.sup.+ and F.sup.-,
there is not evidence of the chemical bonding between the
hydroxyapatite of the bone tissue and the polymethyl methacrylate
of the cement, while on the other hand the fluoride ion has been
shown to have marked tropism for this mineral structure and it is
preferentially captured by it through the dispacement of the
hydroxyl (--OH.sup.-) groups.
In view of the fact that the percentage dry weight of fluorine
present in bone tissue varies physiologically between the level
0.06 and 0.10% and that the safe therapeutic range lies between 021
and 04% it follows that dosing with fluoride salt should aim to
keep the local F.sup.- concentraction within this range.
As a result the change in the mechanical strength properties of the
cement following the addition of fluoride salts in a percentage
adequate to achieve the above-mentioned conditions is negligible,
as has been demonstrated in the laboratory tests.
In fact the range of variation in strength properties (5-10%) lies
well within the limits of the acceptable variation in the mean
values for these properties, these variations beig due for example
to the different compositions of the polymers, or the viscosity
during the polymerisation stage or again the different techniques
of preparation and cementing used.
Other advantages will appear in the course of the following
detailed description of a number of embodiments of the invention
described below by way of non-restrictive examples of the
invention.
With reference to the orthopaedic dose of cement required to attach
a prosthesis to the hip the solid phase of the bone cement
according to the invention consists of 40 g of powder having the
following coposition:
______________________________________ polymetyl methacrylate
(--(C.sub.5 H.sub.8 O.sub.2).sub.n --) 97% benzoyl peroxide
(C.sub.14 H.sub.10 O.sub.4) 3%
______________________________________
The liquid phase on the other hand consists of 14 ml of the
following composition:
______________________________________ monomethyl methacrylate
(C.sub.5 H.sub.8 O.sub.2) 99.10% N--N-dimethyl-p-toluidine (C.sub.9
H.sub.13 N) 0.89% hydroquinone .about.20 ppm
______________________________________
We provide a second example in which the bone cement contains a
fluoride salt, and in this case the composition of the solid phase
is as follows:
______________________________________ sodium fluoride (NaF) 5%
polymetyl methacrylate (--(C.sub.5 H.sub.8 O.sub.2).sub.n --) 92.3%
benzoyl peroxide (C.sub.14 H.sub.10 O.sub.4) 2.7%
______________________________________
The composition of the liquid phase on the other hand is identical
to that in the previous example.
The experimental tests performed in the laboratory have considered
the various types of the bone cement available commercially, in
addition of course to the cement according to the invention.
The instruments used to obtain the data given below in the
corresponding tables were:
for photographic documentation an OPTIPHOT-M microscope provided
with a NICON MICROFLEX FX photographic system,
for particle size documentation a SYMPATEX laser granulometer.
The types of orthopaedic cement powder used were as follows: sample
No. 1 from the CMW 1 company, Sample No. 2 from the SYMPLEX
company, sample No. 3 according to the invention, sample Nos. 4 and
5 obtained in the laboratory by varying the particle size of the
powders.
From an investigation using the microscope and the laser
granulometer and laboratory tests on standard 40 g doses of powder
it was found that:
Sample No. 1 consisted morphologically of a powder comprising a few
spheres, a certain number of spheroids of irregular shape, of
dimensions similar to the said spheres, and amorphous powder.
22 ml of liquid monomer were required in orter to obtain a paste
having a certain degree of worability.
From the point of view of mechanical strength properties the
standard tests performed in accordance with British Standard BS
3531 (Part 7) demonstrated that this sample conformed to the values
by the tests.
As far as particle size is concerned it should be noted that the
percentage passing the 0.90 .mu.m optical sieve was 3.10%, the
10.50 .mu.m sieve 16.87%, the 103.00 .mu.m sieve 100%. The calculed
specific surface area was 0.127 m.sup.2 /cm.sup.3.
Sample No. 2 from the morphologica point of view was in the form of
a powder consisting of a number of spheres with amorphous powder,
there being a complete lack of spheroids.
The amount of liquid monomer absorbed was 20 ml.
The mechanical strength properties of the said sample were in
accordance with the values required by the standards mentioned.
From the particle size analysis data it was found that the
percentage passing the 0.90 .mu.m optical sieve was 2.38%, the
10.50 .mu.m sieve 25.23%, the 103.00 .mu.m sieve 100%.
The specific surface area was 0.122 m.sup.2 /cm.sup.3.
Sample No. 3, which was an embodiment of this invention, appeared
from the morphological point of view as a powder consisting
essentially of perfectly spherical particles of various sizes,
being absultely free of particles produced by grinding of the
polymer.
The amount of liquid phase absorbed was 14 ml.
The mechanical strength properties of the sample of bone cement
according to the invention were in accordance with the values
required by the BS standards.
From the particle size analysis data it was found that the
percentage passing the 0.90 .mu.m sieve was 1.2%, the particle size
classes of 2.60, 3.10 and 3.70 .mu.m were lacking completely, the
percentage passing the 10.50 .mu.m sieve was 6.68%, while the
percentage passing the 87.00 .mu.m sieve was 100%. The specific
surface area was 0.061 m.sup.2 /cm.sup.3.
Sample No. 4 appeared from the morphological point of view as a
powder consisting solely of spheres, amorphous powder and spheroids
being completely lacking.
The amount of the liquid phase absorbed was 13 ml.
The mechanical strength properties of this sample reached the
minima specified by the test.
From the particle size data will be seen that the particle size
classes up to the 5.00 .mu.m sieve are completely lacking, the
percentage passing the 10.50 .mu.m sieve was only 0.51%, while 100%
passed the 103.00 .mu.m sieve. The specific surface area was 0.022
m.sup.2 /cm.sup.3.
Sample No. 5 appeared from the morphological point of view as a
powder consisting solely of spheres, amorphous powder and spheroids
thus being completely absent.
The amount of the liquid phase absorbed was 13 ml.
The mechanical strength properties of this sample did not reach the
minima specified by the test.
From the particle size analysis data will be seen that the shape of
the distribution in similar to that of sample No. 4, in fact the
particle size classes up to the 4.30 .mu.m sieve were completely
missing, the percentage passing the 10.50 .mu.m sieve was only
1.31%, while 100% passed the 103.00 .mu.m sieve. The specific
surface area was 0.025 m.sup.2 /cm.sup.3.
For ease of the comparison the more significant data concerning the
particle size analysis made on the five sample investigated are
shown in Table I.
TABLE I ______________________________________ SAMPLE No. 1 2 3 4 5
______________________________________ SIEVE from 0,90 .mu.m 3,40%
2,38% 1,13% 0,00% 0,00% 0,91 .div. 2.20 .mu.m 4,84% 3,71% 1,13%
0,00% 0,00% 2,21 .div. 3,70 .mu.m 1,92% 2,71% 0,03% 0,00% 0,00%
3,71 .div. 10,50 .mu.m 6,71% 16,43% 4,39% 0,55% 1,31% 10,51 .div.
25,0 .mu.m 15,85% 24,00% 17.64% 4,75% 9,32% 25,01 .div. 51,0 .mu.m
42,66% 27,87% 49,76% 39,59% 42,66% 51.01 .div. 73.0 .mu.m 20,09%
16,88% 22,10% 38,13% 34,85% 73,01 .div. 87,0 .mu.m 4,28% 4,82%
3,82% 12,42% 9,66% over 87,01 .mu.m 0,26% 1,21% 0,00% 4,55% 2,20%
SPECIFIC 0,12749 0,12246 0,05935 0,021894 0,02552 SURF.AREA m.sup.2
/cm.sup.3 MONOMER 22 20 14 13 13 ABSORBED by one dose ml
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As mentioned earliesr all five types of cement were subjected to
copression tests using test pieces prepared under the same
environmental conditions and using an appropriate cylindrical press
having a diameter of 25 mm and a height of 10 mm as specified in
the British standard mentioned.
All the test pieces were prepared the day before the test and the
test procedures were in accordance with the requirements of the
standards specified.
In Table II below we show mean values for the compressive strength,
each relating to 20 test pieces, obtained as the ratio of the yield
strength to the cross-sectional area of the test piece.
TABLE II ______________________________________ SAMPLE No. 1 2 3 4
5 ______________________________________ COMPRESSIVE 84,5 89.0
106,0 65,0 60.0 STRENGTH M Pa MINIMUM COMPRESSIVE STRENGTH: 70,0 M
Pa (according to standard BS 3531)
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From a comparison of the above samples it is clear that a precise
choice of the polymer powder which is to be used to obtain the bone
cement should be made from the point of view of both morphology and
particle size.
If in fact the choice falls, as in the case of the samples 1 and 2,
on a type of powder consisting of a mixture of polymer in the form
of spheres of various sizes mexed with amorphous powder and/or
irregular spheroids, there will be two main consequences from such
a choice:
(a) the powder will have to absorb an appreciable quantity of
liquid monomer in order to attain standard workability.
(b) orthopaedic cements having mechanical strength properties
greater than the limits imposed by BS acceptance standards will be
obtained.
If on the other hand the choice falls on the type of powder
consisting only of spheres of almost equal diameter, as in the case
of samples 4 and 5, or which in any event do not respect certain
proportions between the percentages passing the various optical
sieves, the consequences from this choice will be as follow:
(a) the powder will absorb a minimum amount of liquid monomer in
order to reach a given standard workability.
(b) orthopaedic cements with mechanical strength properties below
the limits specified by BS acceptance standards will be
obtained.
It will be seen therefore that in the first case advantages are
derived from the satisfactory mechanical properties of the cement,
but not all the disadvantages resulting from the presence of an
excessive amount of liquid monomer, i.e. high polymerisation
temperature and cardio-respiratory shoch, will not be
eliminated.
In the second case however the disadvantages due to the excessive
amount of liquid monomer are avoided but the mechanical strength
properties are not sufficient to ensure that the artificial
prosthesis implant will ultimately prove satisfactory.
In the case of sample 3, i.e. in the case of the cement according
to the invention, the choice of a particular type of powder with
the said morphological and particle size characteristics achieves
both advantages together, both those resulting from the reduced
amount of liquid monomer and those resulting from the optimum
mechanical strength properties.
From what has been said so far it is clear that the selection of a
polymer powder consisting of spheres only is only valid if
attention is paid to both the particle sizes and the relative
percentages of the various fractions passing through the sieve.
In fact the presence of a fraction of 1.13% passing through the
0.90 .mu.m sieve has a very important part to play, that is to fill
the empty spaces left by the larger particles when they are in
contact. This makes it possible to obtain a more compact and
therefore stronger cement which is therefore suitable for
orthopaedic use.
If this fraction is larger, for example in excess of 2.00%, as in
the case of sample 1 and 2, surface effects predominate and the
amount af the liquid monomer would have to be increased in order
that the entire paste should react, and in order to achieve the
required degree of workability. This phenomenon is even more marked
if the particles are no larger spherical and therefore have a high
specific area.
If the said fraction is completely absent, as in the case of sample
4 and 5 the spaces between the larger particles will be filled by
monomer only and the final result will be a friable cement
unsuitable for orthopaedic use, despite the fact that the
polymerisation temperature will lie within harmless limits.
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